This can have multiple reasons, the problem we had recently that caused this
error was with IE8, the problem was that the reverse DNS entry was not
pointing to the same name. In other words, we would access the server (called
SPServ) using http://sharepoint, however, the reverse DNS entry would point
to SPServ instead of "sharepoint".
Other reasons for this problem are also possible, but for these problems
more information is needed:
- What Browser (incl. version) is being used?
- What is the protocol (http or https)?
- DNS Entry used